1. Field of the Invention
The present invention relates to a method for controlling an array antenna capable of changing a directivity characteristic of an array antenna apparatus including a plurality of antenna elements. In particular, the invention relates to a method for controlling an array antenna capable of adaptively changing an directivity characteristic of an electronically steerable passive array radiator (ESPAR) antenna (hereinafter, referred to as an ESPAR antenna) equipped with a single radiating element and a plurality of parasitic elements.
2. Description of the Related Art
An ESPAR antennas of related art is proposed in, for example, U.S. Pat. No. 6,407,719, the Related art document 1 of T. Ohira et al., xe2x80x9cElectronically steerable passive array radiator antennas for low-cost analog adaptive beamformingxe2x80x9d, 2000 IEEE International Conference on Phased Array System and, Technology pp. 101-104, Dana point, Calif., May 21-25, 2000, and the Japanese Patent Laid-Open Publication No. 2001-24431. This ESPAR antenna is equipped with an array antenna which includes a radiating element to which a radio signal is fed, at least one parasitic element which is provided apart by a predetermined distance from the radiating element and to which no radio signal is fed, and a variable-reactance element connected to the parasitic element, where the directivity characteristics of the array antenna can be changed, by changing the reactance value of the variable-reactance element.
A beamforming method using spatial power combining, such as that in the ESPAR antenna, is capable of achieving a variable directivity, and this leads to obtaining a high gain, with a simple hardware configuration and low power consumption. Therefore, an antenna of this method can be expected as a practical terminal-mounted adaptive antenna (in particular, one mounted on a mobile user terminal).
However, in the case of the ESPAR antennas, it is impossible to observe any signal on a passive element. Therefore, it is necessary to observe only an output signal from a single port and process the output signal as a feedback signal for adjusting the reactance value. In other words, most methods prepared for conventional adaptive arrays cannot be directly applied to the ESPAR antenna.
In order to solve this problem, there has been proposed, for example, in the Japanese Patent Laid-Open Publication No. 2002-118414, a control method (hereinafter, referred to as first related art method) for, by using the so-called xe2x80x9csteepest gradient methodxe2x80x9d, perturbing the reactance values of respective variable-reactance elements sequentially by a predetermined shift amount, calculating a gradient vector for a predetermined estimation function value versus respective reactance values, and calculating and setting, based on the calculated gradient vector, reactance values of the respective variable-reactance elements, thereby directing the main beam of the array antenna toward a desired wave and directing the null(s) thereof toward the direction(s) of the interference wave(s) so that the estimation function value becomes the maximum or the minimum thereof.
However, this first related art method involves successive perturbations in order to determine respective components of the gradient vector, which in turn involves (M+1) times of calculations of an objective function in each iteration of perturbation. In the case of the ESPAR antenna, it is necessary to provide a training sequence at least (M+1) times longer than that of conventional adaptive arrays, and this leads to such a problem as increase in calculation quantity.
Also, with the use of the first related art method, since a relatively large amount of trials is required for pursuing an optimum solution, and this leads to such a problem as longer convergence time.
On the other hand, in the Related art document 2 of Yukihiro Kamiya et al., xe2x80x9cPerformance consideration for the ESPAR antennaxe2x80x94Statistical considerations of SINR characteristics based on the random weight searchxe2x80x94xe2x80x9d, Technical Report of IEICE, A-P 2000-175, SANE2000-156, pp. 17-24, published in January, 2001 by the Institute of Electronics, Information and Communication Engineers in Japan (IEICE), the following procedure of xe2x80x9cRandom Search Methodxe2x80x9d (hereinafter, referred to as second related art method) is used:
(1) Given a column vector x whose elements are reactance values of respective variable-reactance elements, the column vector x is formed as a reactance matrix. Such a matrix is generated by uniform random numbers within a predetermined range, thereby generating a population of reactance matrices;
(2) Reactance matrices contained in the generated population are loaded to the ESPAR antenna one by one, where samples of received signals are observed in the respective cases, and a predetermined cross correlation coefficient between the received signals and a training sequence signal is calculated;
(3) A reactance matrix that gives the largest cross correlation coefficient among the obtained plurality of cross correlation coefficients is adopted as a weight coefficient.
This second related art method involves only one-time calculation of the cross correlation coefficient for each iteration. However, this method has such a problem that since the succeeding trial is independent of the preceding trial, nothing has been trained when a trial is completed.
A first object of the present invention is to provide a method for controlling an ESPAR antenna, capable of solving the above problems, that is, a method for controlling an array antenna which does not require any long training sequence signal and which allows the performance to be improved every iteration of search, as compared with the related art methods, for directing the main beam toward a desired wave and directing the null(s) thereof toward an interference wave(s).
Also, a second object of the invention is to provide a method for controlling an ESPAR antenna which has solved the above problems, that is, a method for controlling an array antenna, capable of remarkably reducing the convergence time as compared with that of the related art methods, and capable of achieving adaptive control so as to direct the main beam toward a desired wave and to direct the null(s) thereof toward an interference wave(s), with less calculation amount.
Further, a third object of the invention is to provide a method for controlling an ESPAR antenna capable of solving the above problems, that is, a method for controlling an array antenna, capable of obtaining a successful estimation function value and obtaining a successful convergence value at a higher speed with less iterations, as compared with those of the related art methods, for directing the main beam toward a desired wave and directing the null(s) thereof toward an interference wave(s).
According to the first aspect of the present invention, there is provided a method for controlling an array antenna, the array antenna including:
a radiating element for receiving a radio signal;
a plurality of parasitic elements provided apart from the radiating element by a predetermined distance;
a plurality of variable-reactance elements connected to the plurality of parasitic elements, respectively; and
controlling means for changing a directivity characteristic of the array antenna by changing each reactance value set to each of the variable-reactance elements so that each of the parasitic elements operates as either one of a director and a reflector,
wherein the method includes a step of iterating the following steps of:
upon setting the reactance values of the respective variable-reactance elements by randomly perturbing the reactance values from predetermined initial values, calculating predetermined cross correlation coefficients between a received signal and a training sequence signal before and after the perturbation, the received signal being obtained by receiving by the array antenna a training sequence signal contained in a radio signal transmitted from a remote transmitter, and the training sequence signal being generated so as to have a signal pattern identical to that of the transmitted training sequence signal;
selecting and setting reactance values when the cross correlation coefficient increases between those before and after the perturbation; and
setting reactance values obtained by randomly perturbing the selected reactance values, to the variable-reactance elements, respectively.
Also, according to the second aspect of the present invention, there is provided a method for controlling an array antenna, the array antenna including:
a radiating element for receiving a radio signal;
a plurality of parasitic elements provided apart from the radiating element by a predetermined distance;
a plurality of variable-reactance elements connected to the plurality of parasitic elements, respectively; and
controlling means for changing a directivity characteristic of the array antenna by changing each reactance value set to each of the variable-reactance elements so that each of the parasitic elements operates as either one of a director and a reflector,
wherein the method includes:
upon dividing a range of each reactance value available for each of the variable-reactance elements, into two ranges thereof and setting representative values of respective divided two ranges to the variable-reactance elements, respectively, a first step of calculating predetermined cross correlation coefficients between a received signal and a training sequence signal before and after the perturbation, the received signal being obtained by receiving by the array antenna a training sequence signal contained in a radio signal transmitted from a remote transmitter, and the training sequence signal being generated so as to have a signal pattern identical to that of the transmitted training sequence signal, and selecting and setting, as initial values, reactance values of the respective variable-reactance elements corresponding to a larger cross correlation coefficient out of the two cross correlation coefficients corresponding to the representative values of the respective divided two ranges; and
a second step of dividing a range belonging to the selected reactance values into two ranges thereof, calculating the cross correlation coefficients upon setting of the representative values of the respective divided two ranges to the variable-reactance elements, respectively, and selecting and setting reactance values of the variable-reactance elements corresponding to a larger cross correlation-coefficient out of the two cross correlation coefficients corresponding to the representative values of the respective divided two ranges,
thereby controlling a main beam and a null(s) of the array antenna so that the main beam is directed toward a desired wave and the null(s) is directed toward an interference wave(s).
Further, according to the third aspect of the present invention, there is provided a method for controlling an array antenna, the array antenna including:
a radiating element for receiving a radio signal;
a plurality of parasitic elements provided apart from the radiating element by a predetermined distance;
a plurality of variable-reactance elements connected to the plurality of parasitic elements, respectively; and
controlling means for changing a directivity characteristic of the array antenna by changing each reactance value set to each of the variable-reactance elements so that each of the parasitic elements operates as either one of a director and a reflector,
wherein the method includes a control step of:
perturbing the reactance values of the variable-reactance elements, respectively, by a predetermined step width, sequentially, calculating a predetermined estimation function value for each of the reactance values, setting post-perturbation values to the reactance values when the estimation function values calculated for each of the variable-reactance elements before and after, the perturbation are improved whereas setting pre-perturbation values to the reactance values when the estimation function values calculated before and after the perturbation are not improved, decreasing the step width for a succeeding-iteration process with respect to a reactance value of a variable-reactance element for which the estimation function value is not improved, and further iteratively executing a process of inverting a sign of the step width,
thereby calculating and setting reactance values of the variable-reactance elements, respectively, for directing a main beam of the array antenna toward a desired wave and directing a null(s) thereof toward an interference wave(s).
Still further, according to the fourth aspect of the present invention, there is provided a method for controlling an array antenna, the array antenna including:
a radiating element for receiving a radio signal;
a plurality of parasitic elements provided apart from the radiating element by a predetermined distance;
a plurality of variable-reactance elements connected to the plurality of parasitic elements, respectively; and
controlling means for changing a directivity characteristic of the array antenna by changing each reactance value set to each of the variable-reactance elements so that each of the parasitic elements operates as either one of a director and a reflector,
wherein the method includes the following steps of:
perturbing the reactance values of the variable-reactance elements, respectively, by a predetermined difference width xcex94X sequentially, calculating a predetermined estimation function value for each of the reactance values, and based on the calculated estimation function value and by using a steepest gradient method having a step width xcexc, iteratively calculating reactance values of the variable-reactance elements, respectively, so that the estimation function value becomes either one of the maximum and the minimum; and
upon calculating and setting each of the reactance values of the variable-reactance elements for directing a main beam of the array antenna apparatus toward a desired wave and directing a null(s) thereof toward an interference wave(s), decreasing the difference width xcex94X and the step width xcexc by using a predetermined decreasing function depending either one of on the estimation function value f and on a signal to interference noise ratio SINR calculated from the estimation function f.
Still further, according to the fifth aspect of the present invention, there is provided a method for controlling an array antenna, the array antenna including:
a radiating element for receiving a radio signal;
a plurality of parasitic elements provided apart from the radiating element by a predetermined distance;
a plurality of variable-reactance elements connected to the plurality of parasitic elements, respectively; and
controlling means for changing a directivity characteristic of the array antenna by changing each reactance value set to each of the variable-reactance elements so that each of the parasitic elements operates as either one of a director and a reflector,
wherein the method includes the following steps of:
calculating a predetermined estimation function value based on the received radio signal, calculating difference reactance values of the variable-reactance elements, respectively, based on the calculated estimation function value by using a Marquardt method having a predetermined Marquardt number, perturbing the reactance values of the respective variable-reactance elements by a predetermined difference reactance value sequentially, and iterating the above steps, thereby calculating and setting optimum solutions of reactance values of the variable-reactance elements for directing a main beam of the array antenna toward a desired wave and directing a null(s) thereof toward an interference wave(s), so that the estimation function value becomes either one of the maximum and the minimum.